Abstract This paper focuses on the coupling between an ex–tropical cyclone and two preexisting mesoscale tropopause depressions (TDs). The TDs approached the cyclone from widely separated sources after becoming cut off from different upper-level troughs upstream. The first part of the paper combines Meteosat imagery with products from the limited-area version of the operational U.K. Meteorological Office Unified Model to reveal the 3D structure and evolution of the mesoscale features. Each TD was a potential vorticity (PV) maximum characterized by dry-intrusion air descending slantwise beneath an upper-level jet streak. Each TD generated its own cloud head and each is believed to have contributed to the deepening of the surface cyclone. Later parts of the paper identify errors in model forecasts and attribute them to analysis errors in the position of one of the TDs. Two methods are used to locate the analysis errors. Both are capable of being used in real time. The first is the identification via satelli...